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Fast and precise, flat and wide

Telecentric Lenses

Telecentric lenses are designed specifically for use in specialist measurement
applications where per-spective projections and incorrect image scaling can
cause problems. They are particularly suited to i-maging 3D objects where
scaling can often be misinterpreted.

These lenses do not suffer from distortion problems, as they collimate the
light that enters the lens. This results in equal magnification, independent
of object distance and thus no perspective is encountered within the depth of
field around the fixed operating distance. As a consequence of collimating the
light, the aperture of the lens needs to be the same size as the FOV, thus
lenses with a large FOV are physi-cally large and can be very expensive.

For the most demanding measurement applications it is also possible to get a
double sided telecentric lens. This helps to maintain accurate measurements,
even when the image starts to move out of focus, while increasing the depth of
field further, and providing even lower distortion.

Example 1:

This image shows an electrical assembly that
needs to be inspected for damage. As can be
clearly seen, one of the pins is bent, and the
imaging system needs to locate this fault. The
use of a standard "endocentric" lens will
provide an image with perspective distortion,
making the job of detecting the problem
difficult.

This image shows the pins apparently 'fanning
out' from the central axis of the lens. Under
these circumstances, the bent pin appears very
similar to the good pins and presents a much
harder challenge to the vision software.

The next image
shows the same component when viewed through a telecentric
lens. All the components except the bent pin
now appear perpendicular to the lens, with no
perspective distortion. The damaged pin is
now revealed very clearly, making the job of
recognition much easier.

Example 2:

Another application where telecentric lenses have proved very useful is
in the inspection of drawn wire. The gauge of the wire must be checked
very accurately as it leaves the die. However, due to the nature of the
process, a resonance often occurs in the wire which causes its position
to fluctuate and this makes conventional lensing insufficiently accurate.

If a standard lens is used, the distance from the wire to the lens is
constantly changing and hence the apparent width or gauge of the
wire. An important feature of telecentric imaging is that, as the target
moves closer or further away, the size if the image projected onto the
sensor remains the same. This means that no matter where the wire is in relation to the lens, the width remains the same. In this application,
using a telecentric backlight will increase the accuracy of the inspection.

Example 2

This image shows the wire, both
without blurring and also correctly sized,
making it easy for the image analysis to gauge
the diameter accurately.

This is the wire viewed through a standard, not telecentric lens.
There is an obvious size difference because of
the change in working distance and also a
corresponding focusing problem, as the wire
moves up and down in the field of view.

This is the wire viewed from a different position.
There is an obvious size difference because of
the change in working distance and also a
corresponding focusing problem, as the wire
moves up and down in the field of view.

The next diagram illustrates the reason why telecentric lenses are often
so large. Using the same component as a target for both lens types, the
"endocentric" lens collects the light rays from the
object in an angle. The telecentric lens, however,
only collects parallel or collimated rays that
originate from the surface of the target and so
the front aperture must be at least the size
of the object. As lenses can only be
produced up to a certain size, there are
limitations for applications where
larger objects have to be imaged.